Grey matter contains most of the brain’s 100 billion nerve cells, while the white matter is filled with nerve fibres that connect the brain regions.

A study of 9,652 middle-aged people, conducted at the Loughborough University, measured body mass index (BMI) and waist-to-hip ratio. It was found that nearly one in five of the participants were found to be obese.

The findings of the study appeared in the Journal of Neurology.

Researchers also used an MRI to scan participants’ brain volume. The researchers factored in age, physical activity, smoking and high blood pressure, all of which might lead to reduced volume.

The study found that 1,291 people who had a BMI of 30 or higher and a high waist-to-hip ratio had the lowest average grey matter volume, at 786 cubic centimetres; 514 people with a BMI of 30 or higher but without central obesity had an average grey matter volume of 793 cubic centimetres. Meanwhile, 3,025 people with overall health scores had an average grey matter volume of 798 cubic centimetres.

The study also showed no real differences in white matter brain volume linked to obesity. However, excess weight was associated with shrinkage in specific regions of the brain: the pallidum, nucleus accumbens, putamen (linked only to a higher BMI) and caudate (linked only to a higher waist-to-hip ratio). All of these brain regions are involved in motivation and reward.